Governor Christie said Tuesday that he will close a looming gap in New Jersey’s budget by pulling back nearly $900 million that had been earmarked for the state’s teetering pension system, reversing his earlier promises to increase those pension contributions and provoking further scrutiny of the state’s credit rating.

The ailing pension system — and the need to prop it up with more and larger payments — was cited as a key factor by all three major Wall Street ratings agencies when they lowered New Jersey’s credit rating in recent weeks, making it potentially more expensive for the state to borrow money. It was the sixth such downgrade since 2010, giving New Jersey one of the lowest credit ratings in the country at a time when the overall state economy also struggles with an unemployment rate that is higher than the national average.

But Christie, during a State House news conference, said New Jersey cannot afford to make the pension contributions he had agreed to when he signed a 2010 law mandating them for the next seven years. He is also asking lawmakers to authorize an additional $1.5 billion reduction in the state pension contribution for the fiscal year that begins July 1.

“Our problem is that we have made promises to people that we cannot keep, so we have to adjust this going forward,” the governor said, referring to what he has called overly generous employee benefits granted by previous administrations.

For Christie, the pension reversal could undercut one of the accomplishments he has been highlighting in his own résumé during his rise to national prominence in the Republican Party.

In speeches throughout the country underscoring his leadership skills, he has repeatedly pointed to a series of givebacks he persuaded Democratic lawmakers to approve in 2011 — including higher employee contributions and an end to cost-of-living increases — as helping to right the pension system.

Christie actually skipped the pension contribution altogether during his first year in office, but then committed to the stepped-up payments starting in 2011.

But even those promised payments represented only a fraction of what actuaries say the state should be putting into the system each year to bring it back to solvency.

Financial experts have warned that, without full payments or further reforms, the fund might not have enough money to cover its obligations to retirees in 15 to 20 years.

Now Christie is conceding even those phased-in contributions, totaling $1.6 billion this fiscal year and $2.25 billion during the next one, are too costly.

The New Jersey pension system, currently valued at $76.76 billion, is one of the nation’s largest, with nearly 800,000 receiving benefits. But after years of skipped or partial state payments, it is also grossly underfunded, by roughly $52 billion, counting both state and local government obligations.

Those liabilities were cited by all three major ratings agencies in their assessment of New Jersey’s credit rating, which is a central factor in determining how cheap and easy it is for the state to borrow money for items like schools and bridges that are too expensive to be paid for with available cash.

Fitch Ratings senior director Marci Block said Tuesday she was hoping to learn more about Christie’s budget plans when the state treasurer appeared before lawmakers this week.

“At this point, that’s kind of where we are,” she said.

Baye Larsen, senior analyst for Moody’s Investors Service, said Christie’s decision to reduce the pension payment is “adding to next year’s gap and future years’.”

Moody’s warned last week that using more “non-recurring or uncertain budget solutions” could lead to another downgrade.

Christie also called on lawmakers to consider new changes to public-employee benefits, and he blamed the decisions by previous governors to skip pension payments as helping to drive up the costs he’s now facing.

“We will not make the payments that are applied to the sins of the past,” he said.

Democrats have argued that Christie’s refusal to raise taxes or cut state services, coupled with his administration’s failure to accurately project revenues in the $33 billion spending plan he signed last June, led to the current woes.

Christie’s announcement sets up a new battle with those Democratic legislative leaders, including Senate President Stephen Sweeney, who threatened earlier this year to shut down state government if Christie reneged on the pension contribution. The governor doesn’t need lawmakers’ approval to cut the pension payment in the current fiscal year, but he will need their votes to adopt a budget for the fiscal year that begins July 1.

Christie originally proposed a $34.4 billion spending plan for the new fiscal year, but the treasurer is expected to announce a scaled-back version when he appears before lawmakers Wednesday.

Sweeney, D-Gloucester, issued a statement Tuesday that didn’t repeat the threat to shut down the government, promising instead that “Democrats in the Legislature will work to resolve this issue in a manner that is fair to the hardworking, middle-class people of this state.” He offered no specifics.